1. Field of the Invention
The present invention relates generally to the fields of blood vessels and tumor biology. More particularly, it embodies the surprising findings that aminophospholipids, such as phosphatidylserine and phosphatidylethanolamine, are accessible, stable and specific markers of tumor vasculature. The invention thus provides therapeutic constructs and conjugates that bind to aminophospholipids for use in delivering toxins and coagulants to tumor blood vessels and for inducing thrombosis and tumor regression.
2. Description of the Related Art
Tumor cell resistance to chemotherapeutic agents represents a significant problem in clinical oncology. In fact, this is one of the main reasons why many of the most prevalent forms of human cancer still resist effective chemotherapeutic intervention, despite certain advances in the field of chemotherapy.
A significant problem to address in tumor treatment regimens is the desire for a "total cell kill". This means that the more effective treatment regimens come closer to a total cell kill of all so-called "clonogenic" malignant cells, i.e., cells that have the ability to grow uncontrolled and replace any tumor mass that might be removed by the therapy. Due to the goal of developing treatments that approach a total cell kill, certain types of tumors have been more amenable to therapy than others. For example, the soft tissue tumors, e.g., lymphomas, and tumors of the blood and blood-forming organs, e.g., leukemias, have generally been more responsive to chemotherapeutic therapy than have solid tumors, such as carcinomas.
One reason for the susceptibility of soft and blood-based tumors to chemotherapy is the greater accessibility of lymphoma and leukemic cells to chemotherapeutic intervention. Simply put, it is much more difficult for most chemotherapeutic agents to reach all of the cells of a solid tumor mass than it is the soft tumors and blood-based tumors, and therefore much more difficult to achieve a total cell kill. Increasing the dose of chemotherapeutic agents most often results in toxic side effects, which generally limits the effectiveness of conventional anti-tumor agents.
Another tumor treatment strategy is the use of an "immunotoxin", in which an anti-tumor cell antibody is used to deliver a toxin to the tumor cells. However, in common with the chemotherapeutic approaches described above, immunotoxin therapy also suffers from significant drawbacks. For example, antigen-negative or antigen-deficient cells can survive and repopulate the tumor or lead to further metastases. Also, in the treatment of solid tumors, the tumor mass is generally impermeable to molecules of the size of antibodies and immunotoxins. Both the physical diffusion distances and the interstitial pressure within the tumor are significant limitations to this type of therapy.
A more recent strategy has been to target the vasculature of solid tumors. Targeting the blood vessels of the tumors, rather than the tumor cells themselves, has certain advantages in that it is not likely to lead to the development of resistant tumor cells, and that the targeted cells are readily accessible. Moreover, destruction of the blood vessels leads to an amplification of the anti-tumor effect, as many tumor cells rely on a single vessel for their oxygen and nutrients (Denekamp, 1990). Exemplary vascular targeting strategies are described in U.S. Pat. Nos. 5,855,866 and 5,965,132, which particularly describe the targeted delivery of anti-cellular agents and toxins to protein markers of tumor vasculature.
Another effective version of the vascular targeting approach is to target a coagulation factor to a protein marker expressed or adsorbed within the tumor vasculature (Huang el al., 1997; U.S. Pat. Nos. 5,877,289, 6,004,555 and 6,093,399. The delivery of coagulants, rather than toxins, to tumor vasculature has the further advantages of reduced immunogenicity and even lower risk of toxic side effects. As disclosed in U.S. Pat. No. 5,877,289, a preferred coagulation factor for use in such tumor-specific thrombogens, or "coaguligyands", is a truncated version of the human coagulation-inducing protein, Tissue Factor (TF). TF is the major initiator of blood coagulation (Ruf et al., 1991; Edgington et al., 1991; Ruf and Edgington, 1994). Treatment of tumor-bearing mice with such coaguligands results in significant tumor necrosis and even complete tumor regression in many animals (Huang et al., 1997; U.S. Pat. Nos. 5,877,289, 6,004,555 and 6,093,399.
Although the specific delivery of therapeutic agents, such as anti-cellular agents, toxins and coagulation factors, to protein markers of tumor vessels represents a significant advance in tumor treatment protocols, there is still room for additional vascular targeting therapies. The identification of additional stable targets to allow specific tumor vessel destruction in vivo would naturally be of benefit in expanding the number of targeting options. More particularly, the development of targeting agents for delivering therapeutics even closer to the tumor vascular endothelial cell membrane would represent an important advance.